Install gdb 8 for CI:

A bug in gdb is crashing when demangling some symbols. Gdb needs to be updated
to at least gdb 8.

Only run the debug build under gdb (other builds can time out).

bin/ci/ubuntu/build-and-test.sh

@@ -62,15 +62,19 @@ if [[ $TARGET == "coverage" ]]; then
   lcov --no-external -c -i -d . -o baseline.info
 fi
 
-# Execute unit tests under gdb, printing a call stack
-# if we get a crash.
-gdb -return-child-result -quiet -batch \
-    -ex "set env MALLOC_CHECK_=3" \
-    -ex "set print thread-events off" \
-    -ex run \
-    -ex "thread apply all backtrace full" \
-    -ex "quit" \
-    --args $APP_PATH $APP_ARGS
+if [[ ${TARGET} == debug ]]; then
+    # Execute unit tests under gdb, printing a call stack
+    # if we get a crash.
+    $GDB_ROOT/bin/gdb -return-child-result -quiet -batch \
+                      -ex "set env MALLOC_CHECK_=3" \
+                      -ex "set print thread-events off" \
+                      -ex run \
+                      -ex "thread apply all backtrace full" \
+                      -ex "quit" \
+                      --args $APP_PATH $APP_ARGS
+else
+    $APP_PATH $APP_ARGS
+fi
 
 if [[ $TARGET == "coverage" ]]; then
   # Create test coverage data file